Specifications

4.5.4 MSTP(MultipleSpanningTreeProtocol)Configuration
MSTPistheabbreviationofMultipleSpanningTreeProtocol.Thispr otocolisa
directextensionofRSTP.Itcanprovideanindependentspanningtreefor
differentVLANs.Itsimplifiesnetworkmanagement,providesforevenfaster
convergencethanRSTPbylimitingthesizeofeachregion,andpreventsVLAN
membersfrombeingsegmentedfromtherestofthegroup(assometimesoccurs
withIEEE802.1DSTP).
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WhileusingMSTP,therearesomenewconceptsofnetworkarchitecture.A
switchmaybelongtodifferentgroup,actsasrootordesignateswitch,generate
BPDUforthenetworktomaintaintheforwardingtableofthespanningtree.
WithMSTP,itcanalsoprovidemultipleforwardingpathsandenableload
balancing.Understandthearchitectureallowsyoutomaintainthecorrect
spanningtreeandoperateeffectively.
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OneVLANcanbemappedtoaMultipleSpanningTreeInstance(MSTI).The
maximumInstanceofPMIManagedSwitchsupportis16,rangefrom0‐15.The
MSTPbuildsaseparateMultipleSpanningTree(MST)foreachinstanceto
maintainconnectivityamongeachoftheassignedVLANgroups.AnInternal
SpanningTree(IST)isusedtoconnectalltheMSTPswitcheswithinanMST
region.AnMSTRegionmaycontainmultipleMSTPInstances.
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Thefigureshowsthereare2VLANs/MSTPInstancesandeachinstancehasits
Rootandforwardingpaths.
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ACommonSpanningTree(CST)interconnectsalladjuacentMSTregionsandacts
asavirtualbridgenodeforcommunicationswithSTPorRSTPnodesintheglobal
network.MSTPconnectsallbridgesandLANsegmentswithasingleCommon
andInternalSpanningTree(CIST).TheCISTisformedasaresultoftherunning
spanningtreealgorithmbetweenswitchesthatsupporttheSTP,RSTP,MSTP
protocols.
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